1. Field
The embodiments discussed herein refer to broadband wireless access networks, and more particularly they concern a connection-based scheduling method with differentiated service support for hierarchical multi-hop relay networks. The embodiments can be used for instance in networks based on IEEE standards 802.16x, which is one of the promising standards where protocol elements are defined, worth of being considered when designing air interfaces for new generation systems, i.e. beyond-3G (3rd Generation) and 4G (4th Generation) systems. In this respect, reference can be made to: IEEE 802.16-2004, IEEE Standard for Local and Metropolitan area networks—Part 16: Air Interface for Fixed Wireless Access Systems, October 2004, and IEEE Std 802.16e-2005, Amendment to IEEE Standard for Local and Metropolitan Area Networks—Part 16: Air Interface for Fixed Broadband Wireless Access Systems—Physical and Medium Access Control Layers for Combined Fixed and Mobile Operation in Licensed Bands, February 2006.
2. Description of the Related Art
The very high data rates envisioned for 4G wireless systems in reasonably large areas do not appear to be feasible with the conventional cellular architecture due to two basic reasons. First, the transmission rates envisioned for 4G systems are two orders of magnitude higher than those of 3G systems, and it is well known that for a given transmit power level, the symbol (and thus bit) energy decreases linearly with the increasing transmission rate. Second, the spectrum that will be released for 4G systems will almost certainly be located well above the 2 GHz band used by the 3G systems. The radio propagation in these bands is significantly more vulnerable to non-line-of-sight conditions, which is the typical mode of operation in today's urban cellular communications.
The brute-force solution to this problem is to significantly increase the density of the base stations, resulting in considerably higher deployment costs, which would only be feasible if the number of subscribers also increased at the same rate. This seems unlikely to happen, the penetration of cellular phones and other mobile terminals already being high in the developed countries. On the other hand, the same number of subscribers will have a much higher demand in transmission rates. Since presumably subscribers would not be willing to pay the same amount per data bit as for voice bits, a drastic increase in the number of base stations does not seem therefore economically justifiable.
However, fundamental enhancements are necessary for the very ambitious throughput and coverage requirements of future systems. Towards this end, in addition to advanced transmission techniques and co-located antenna technologies, some major modifications in the wireless network architecture itself are required. The integration of multi-hop relaying capability, by which an effective distribution and collection of signals to and from the wireless users is entrusted not only to the base station but also to other network elements (relays) is perhaps the most promising architectural upgrade for extending the coverage of conventional (single-hop) wireless networks at reasonable costs. A multi-hop hierarchical relay network is a network where a base station is associated with a plurality of Relay Nodes (RNs), arranged e.g. according to a logical tree structure, and last-hop (or single-hop) connections are provided towards user terminals (UTs) around each relay node. The multi-hop traffic is transmitted between the base station, which is connected to a fixed backbone network, and the relay nodes that are strategically placed. The last-hop traffic takes place between the relay node and a variable number of user terminals.
The multi-hop technology allows enlarging the overall system coverage with low cost infrastructures, since the relay nodes have a simpler structure and therefore are cheaper than base stations. However, the task of ensuring Quality of Service (QoS) requirements (throughput, delay, jitter, etc.) becomes more complex.
A resource request and allocation strategy at the Medium Access Control (MAC) level keeping limited the end-to-end multi-hop delay has been proposed in our co-pending European Patent Application No. 05485475.0, filed on 1 Jul. 2005, entitled “Connection based scheduling method for hierarchical multi-hop wireless networks extended to beyond 3G radio interface”. That application represents the closest prior art and claim 1 thereof recites (the parenthetical references to the Figures are omitted):
“Method for controlling the access to a TDMA wireless channel from nodes deployed as either a linear or tree topology network for multihop transmissions in uplink from a requesting node to a centralized node and/or in downlink from the centralized node towards an end node, including the steps of:                issuing network topology information from the centralized to the other nodes;        computing the amount of resources needed on each individual link between adjacent nodes, by the transmitting node on that link;        releasing permissions, also called grants, for the use exclusive of TDMA channel for a given time by the centralized node to each node along uplink and/or downlink multihop path/s, characterized in that said requesting node issues a cumulative request for the resources needed on each link along the end-to-end path.”        
According to that strategy, the requests of resources for sending uplink flows from relay nodes to the base station and/or downlink flows from base station to relay nodes are computed by each requesting node for the end-to-end connection instead of being computed only for the next link towards the destination. This is just the meaning of “connection based scheduling”. This is made possible in networks with tree topology and centralized scheduling where a request of resources is computed on individual links between two adjacent nodes, and the network configuration is generally known to the requesting nodes. In practice, each requesting node issues a cumulative request given by summing up the same request for each link that separates the node from the base station (in uplink) plus each link separating the base station from the destination node (in downlink). The base station, in response to all cumulative requests, grants uplink and/or downlink resources for each link. A grant is intended as an individual permission given to the node for the use exclusive of the common resource (e.g. the TDMA radio channel) for a fraction of time. The cumulative request/grant is made possible, e.g. in IEEE 802.16 networks, by the structure of the centralized control scheduling messages
This strategy, together with an order of transmission depending on the topology (in uplink direction the node farthest form the base station transmits first and the node closest to the base station transmits last, and in downlink direction transmission occurs in the reverse order) guarantees that packets wait for being transmitted only in the source relay nodes and not in the forwarding or transit relay nodes, and that they are delivered to the destination within one frame once they are sent from the source node. A further one-frame delay is to be considered in the average for the last hop from/to the user terminal. Also, fairness in respect of the number of hops and of the propagation direction (as shown by the delay curves reported in FIGS. 16 and 17 of the application) is achieved.
However, this strategy does not take into account that a relay node generally handles connections associated with services having different Quality of Service (QoS) requirements, such as, in the simplest case, real time and non-real time services (e.g. to support both multimedia and web browsing applications). A grant of resources determined on the basis of the total traffic of the relay nodes can result ultimately in a risk of lack of resources for real time traffic (or generally traffic with higher QoS requirements), especially for nodes more distant from the base station: this results in turn in a degradation of the QoS, especially in case traffic distribution among real time/non real time services (or, generally, among different classes of service) at the different nodes is non-uniform.